The formation and reactivity of HOC+: Interstellar implications

Abstract

CHO+ ions are made by electron impact on CD3OH in the source (ICR1) of a tandem ion cyclotron resonance spectrometer. These ions are injected into a differentially pumped analysis cell (ICR2) where they are reacted with a number of small molecules. The internal energy distribution in the CHO+ ions is obtained using total reactivity studies with neutral molecules of varying proton affinities. About 40% of the CHO+ ions react with D2 either by proton transfer to form D2H+ or isotopic exchange to form CDO+ ions. A series of experiments are performed that conclusively show these ions are the HOC+ isomer and the exchange is due to the catalytic isomerization reaction HOC++D2→DCO++HD which is approximately 37 kcal/mol exothermic. The product DCO+ ions are vibrationally cool indicating the reaction releases most of its energy as kinetic energy. Absolute rate constants for reactions of CHO+ ions with the neutrals 13CO, 15N2, CO2, O2, D2, and Ar are reported. HOC+ reacts with D2 at about 30% of the collision rate. This rate decreases with increasing HOC+ kinetic energy in the range 0.025 to 0.44 eV. The two products of the reaction are D2H+/CO and DCO+/HD with DCO+/HD comprising ∼42% of the products for the HOC+ ions formed from CD3OH. Phase space theory calculations are performed to determine the barrier to catalytic isomerization. The data are best fit with a barrier between 0.0 and −0.05 eV relative to the HOC+/D2 asymptotic energy. A barrier of this magnitude yields reaction rate constants at interstellar temperatures of at least 10% of the collision rate constant (i.e., k>1×10−10 cm3/s), and could explain why so little HOC+ is observed in interstellar clouds.

Proton-transport catalysis and proton-abstraction reactions: An ab initio dynamical study of X+HOC+ and XH++CO (X=Ne, Ar, and Kr)

“…[1][2][3][4][5][6][7][8][9][10][11] However, because there is a substantial barrier of about 150 kJ mol Ϫ1 separating the two forms, [1][2][3][4][5][6][7]11 both isomers can be separately observed as stable species in the gas phase. [8][9][10][12][13][14] For example, the microwave spectra of both HOC ϩ and HCO ϩ have been recorded. 12,13 In addition, both HOC ϩ and HCO ϩ are firmly established interstellar species.…”

“…[15][16][17] The barrier for the rearrangement of HOC ϩ to HCO ϩ has been found to be substantially reduced, or in some cases even eliminated, as the result of interaction with an appropriate neutral molecule ͑X͒. 1,10,14,18,11 This is an example of what Bohme has described as catalyzed proton transport, 19 or what may be referred to more generally as ion-transport catalysis. 20 The reason for the barrier lowering is straightforward.…”

Dihydrogen Catalysis: A Remarkable Avenue in the Reactivity of Molecular Hydrogen

“…This reaction is one the most studied in the theory of DHC processes (16, [23][24][25][26][27]. The current view, which is based on the PES and kinetic studies, indicates that the process has a low barrier of activation.…”

“…(2.3.1a,b)) is a classical example of dihydrogen catalyzed proton-transport reaction (16, [22][23][24][25][26][27]:…”

“…(2.1.3)) has been originally suggested for cationic systems(22)(23)(24)(25)(26)(27). However, it occurs also in neutral Downloaded by [George Mason University] at 02:43 28 September 2014 …”